1. Technical Field
The invention relates to stabilized frequency distribution links, suitable for guided wave and free space applications, and immune to degradations resulting from the variations of the dispersive properties of the propagation medium.
2. Background Art
The distribution of stable reference frequencies from a standard to remote locations enables multiple user service from a single source. This function provides considerable economy in the costs of multiple frequency standards, and their operation in stable thermal and magnetic environments. It also enables, for example, experiments requiring phase coherence between two or more stations within a complex, or experiments requiring transfer of stable references to spacecraft.
In most applications the highest attainable stability is required from the standard. It is therefore crucial that any degradations of the reference originating from the distribution link be kept at a minimum. This requirement has necessitated the use of transmission media with inherently high stabilities, and schemes that electronically stabilize residual instabilities. A notable example of stable systems is the optical fiber based frequency distribution link which has achieved the highest stability for reference frequency distribution.
Schemes for the electronic stabilization of the distribution systems are generally based on a comparison of the phase of the input signal at the location of the standard and the phase of the signal at the remote end. This is usually accomplished by returning part of the signal at the remote end to the input point for comparison, thus requiring a two-way transmission of the reference frequencies. Present electronic stabilization schemes are generally based on phase comparisons at transmission and receiving ends thus requiring two-way transmission.